1. Field of the Invention
The present invention relates to a method of manufacturing a rotating anode type X-ray tube and, more particularly, to a method of manufacturing a rotating anode type X-ray tube in which the rotational balance of a rotary structure to which its anode target is fixed is checked and corrected as required.
2. Description of the Related Art
As is known, in a rotating anode type X-ray tube, a disk-type anode target is supported by a rotary structure and a stationary structure that have bearings between themselves and, while the disk-type anode target is rotated at a high speed by energizing a solenoid coil arranged outside a vacuum chamber, an electron beam is emitted from a cathode and bombarded against the surface of the rotating anode target, so that X-rays are emitted from the anode target. Each bearing is constituted by a ball bearing or a hydro-dynamic pressure slide bearing which has a spiral groove formed in its bearing surface and which uses a liquid metal, e.g., gallium (Ga) or a gallium-indium-tin (Ga--In--Sn) alloy, as a lubricant. An example that uses the latter hydro-dynamic pressure slide bearing is disclosed in, e.g., U.S. Pat. No. 4,641,332, U.S. Pat. No. 3,068,885 (Jpn. Pat. Appln. KOKAI Publication No. 60-117531), (Jpn. Pat. Appln. KOKAI Publication No. 2-227948), and U.S. Pat. No. 5,204,890 (Jpn. Pat. Appln. KOKAI Publication No. 5-144396).
An example of a rotating anode type X-ray tube having a hydro-dynamic pressure slide bearing lubricated with a liquid metal has an arrangement as shown in FIGS. 1 to 4. More specifically, in this rotating anode type X-ray tube, a disk-type anode target 11 is coupled to a distal end portion 13a of an anode target support shaft 13, which projects from one end of a cylindrical rotary structure 12, with a pin 14a and a fixing screw 14b. The support shaft 13 is made of a high-melting metal, e.g., molybdenum, and its central portion is hollow in order to decrease heat conduction. A columnar stationary structure 15 is inserted in the cylindrical rotary structure 12, and a flange-type thrust ring 16 is fixed at the lower end portion of the cylindrical rotary structure 12. A lower end portion 15a of the columnar stationary structure 15 is hermetically bonded to a cylindrical glass portion 17a of a vacuum container or chamber 17 through seal rings 15b. The vacuum chamber 17 has a large-diameter metal portion 17c having a corona ring 17b at the coupling portion with the cylindrical glass portion 17a, and surrounding the anode target 11, and an X-ray radiation window 17d formed in part of the large-diameter metal portion 17c. Note that black coating films (not shown) having a heat emissivity of 0.6 or more are formed on the inner and outer surfaces of the large-diameter metal portion 17c of the vacuum chamber 17 in order to effectively dissipate the radiation heat generated by the anode target 11 outside the tube.
A cathode structure 18 is provided to oppose the anode target 11. Two pairs of hydro-dynamic pressure radial slide bearings 19 like those shown in the official gazettes described above, and two pairs of thrust slide bearings 20 are provided in the fitting portions of the cylindrical rotary structure 12 and the columnar stationary structure 15. The two radial slide bearings 19 separated from each other in the direction of the rotation axis have a pair of herringbone pattern spiral grooves 19a and a pair of herringbone pattern spiral grooves 19b formed in the outer circumferential surface of the stationary structure 15. One of the two thrust slide bearings 20 has a circular herringbone pattern spiral groove 20a, as shown in FIG. 3, formed in a stationary structure end face 15c. The other thrust slide bearing 20 has a circular herringbone pattern spiral groove 20b, as shown in FIG. 4, formed in the upper surface of the thrust ring 16 which is in contact with the stepped surface of the lower portion of the stationary structure 15. The surfaces of the respective slide bearings which are in contact with these spiral groove surfaces are mere smooth surfaces. However, spiral grooves may be formed in these surfaces of the respective slide bearings as required. The bearing surfaces of both the rotary structure 12 and the stationary structure 15 keep a bearing clearance of about 20 .mu.m between themselves during operation.
A lubricant reservoir 21 and a lubricant passage 22 are formed in the stationary structure 15. The lubricant reservoir 21 is formed by boring the central portion of the stationary structure 15 in the axial direction. The lubricant passage 22 is formed in the intermediate portion of the stationary structure 15. The rotary structure 12 has the shaft 13, an iron-alloy intermediate cylinder 23 to which the shaft 13 is fixed, an inner cylinder 24 welded to the lower end portion of the shaft 13, and a copper outer cylinder 25. An insulating clearance 26 having a width of about 0.1 to 1 mm in the radial direction is provided between the inner cylinder 24 whose inner surface serves as a bearing surface and the intermediate cylinder 23 coaxially fitted on the outer circumferential surface of the inner cylinder 24. A liquid metal lubricant (not shown), e.g., a Ga--In--Sn alloy, which liquifies at least during operation is applied to the lubricant reservoir 21, the lubricant passage 22, and the bearing clearance.
The base material of the anode target 11 is made of a high-melting metal, e.g., molybdenum, to constitute an annular heat-accumulating portion 27 having a large volume. An X-ray radiation target layer 28 made of tungsten or a tungsten alloy is formed on a surface of the annular heat-accumulating portion 27 opposing the cathode structure 18. A black coating film 27a having a heat emissivity of 0.6 or more is formed on the outer circumferential surface of the annular heat-accumulating portion 27 opposing the large-diameter metal portion 17c of the vacuum chamber 17. The distal end portion 13a of the support shaft 13 integrally coupled to a rotary structure shoulder portion 12a extends through the anode target 11 and is integrally coupled to the anode target 11 with the pin 14a and the fixing screw 14b, as described above.
To operate this X-ray tube, a drive voltage is supplied to a stator 32, arranged outside the vacuum chamber 17 at a position to correspond to the rotary structure 12 and having a solenoid coil, to generate a rotating magnetic field, thereby rotating the anode target 11 at a high speed. An electron beam is emitted from the cathode structure 18 and bombarded against the target layer 28 of the anode target 11, thereby generating X-rays.
It is needless to say that the rotational balance of a rotating unit obtained by integrally forming the anode target 11 and the rotary structure 12 must be adjusted in advance at high precision. For this purpose, before the rotating unit and the stationary structure 15 are sealed in the vacuum chamber 17, the rotational balance of the rotating unit must be checked. If the rotating unit has an imbalance, for example, part of the anode target 11 is cut off by a predetermined amount, as indicated by reference symbol A in FIG. 1, to adjust the rotational balance, and thereafter the rotating unit is assembled in the vacuum chamber 17. If one cutting operation is not sufficient, the rotational balance is repeatedly checked and corrected.
In a conventionally general structure in which the rotary structure is supported by ball bearings, even when the rotational balance is checked while rotating the rotating unit in the air, a said metal lubricant such as silver or lead for the ball bearings is not substantially degraded. However, in a rotating anode type X-ray tube having a hydro-dynamic pressure bearing as described above that uses a very active liquid metal lubricant, e.g., Ga or a Ga alloy, when the liquid metal lubricant fills a small clearance or the like of the bearing and that between the rotary structure 12 and the stationary structure 15, if the liquid metal lubricant is exposed to air by rotating the rotating unit in the air, the surface of the lubricant itself or the bearing surface wetted with the lubricant oxidizes immediately. When this rotating unit is sealed in a vacuum chamber, a correct bearing performance cannot be obtained. Therefore, a very complicated step of checking and adjusting the rotational balance in a vacuum bell-jar, assembling the rotating unit directly in the vacuum chamber, and the like is needed.